1. Field of the Invention
The present invention relates to a layered high frequency filter incorporating a plurality of resonators.
2. Description of the Related Art
With increasing demands for reductions in dimensions and thickness of communications apparatuses conforming to the Bluetooth standard and those for use on a wireless local area network (LAN), techniques for high-density packaging has been required. One of proposals for meeting such a requirement is to integrate components through the use of a layered substrate.
One of components of the above-mentioned communications apparatuses is a band-pass filter that filters reception signals. As the band-pass filter, a layered band-pass filter such as the one disclosed in Japanese Published Patent Application (hereinafter referred to as “JP-A”) 2000-22404 is known. The layered band-pass filter incorporates a plurality of resonators formed using conductor layers of a layered substrate. In the layered band-pass filter, respective adjacent ones of the resonators are inductively coupled to each other. For the layered band-pass filter, as disclosed in JP-A 2000-22404, there are cases in which the respective adjacent ones of the resonators are also capacitively coupled to each other. In such cases, it is possible to adjust the frequencies of two attenuation poles and the pass-band width of the band-pass filter by adjusting the magnitude of the inductive coupling and the magnitude of the capacitive coupling. Adjustment of the characteristics of the band-pass filter is thus made easier by capacitively coupling the respective adjacent ones of the resonators to each other, compared with a case in which the respective adjacent ones of the resonators are not capacitively coupled to each other.
JP-A 2000-22404 discloses a technique of capacitively coupling the respective adjacent ones of resonators through the use of a coupling adjusting electrode. The coupling adjusting electrode is opposed to each of two adjacent resonators with a dielectric layer disposed in between.
Japanese Published Utility Model Application (hereinafter referred to as “JP-U”) 5-78003 discloses a layered dielectric resonator incorporating a plurality of coil conductors that serve as transmission lines. In this resonator, respective adjacent ones of the coil conductors are opposed to each other with a dielectric layer disposed in between so as to capacitively couple the respective adjacent ones of the coil conductors to each other.
According to the technique disclosed in JP-A 2000-22404, the coupling adjusting electrode is opposed to each of two adjacent resonators with a dielectric layer in between. Consequently, according to this technique, a capacitor is formed between one of the resonators and the coupling adjusting electrode, and another capacitor is formed between the other of the resonators and the coupling adjusting electrode. These two capacitors are connected to each other in series. The respective adjacent two of the resonators are capacitively coupled to each other through such two capacitors connected to each other in series.
According to the technique disclosed in JP-A 2000-22404, the composite capacitance of the two capacitors connected to each other in series is smaller than the capacitance of each of the capacitors. Therefore, in this technique, to make the composite resistance be of a desired value, it is necessary that the area of a region required for forming each of the capacitors, that is, the area of the region in which the coupling adjusting electrode and each of the resonators are opposed to each other, be great to some extent. According to this technique, it is therefore difficult to reduce the size of the filter.
In a layered band-pass filter, it is possible to capacitively couple the respective adjacent two of the resonators to each other through the use of the technique disclosed in JP-U 5-78003. However, this case has a problem that will now be described. In layered band-pass filters, there are some cases in which, when a layered substrate is fabricated, the positional relationship among a plurality of conductor layers disposed at different locations in the direction in which the layers are stacked deviates from a desired positional relationship. This will be hereinafter called displacement of the conductor layers. According to the technique disclosed in JP-U 5-78003, since the two coil conductors are disposed at different locations in the direction in which the layers are stacked, there is a possibility that the relative positional relationship between the coil conductors may vary. If the relative positional relationship between the coil conductors varies, the magnitude of inductive coupling and the magnitude of capacitive coupling between the two coil conductors both vary. Therefore, in the case in which the respective adjacent two of the resonators of the layered band-pass filter are capacitively coupled to each other through the use of the technique disclosed in JP-U 5-78003, the magnitude of inductive coupling and the magnitude of capacitive coupling between adjacent two of the resonators both vary if the relative positional relationship between the two resonators varies due to displacement of the conductor layers. Therefore, this case has a problem that variations in characteristics of the band-pass filter are likely to increase due to the displacement of the conductor layers.
Furthermore, in the case in which the magnitude of inductive coupling and the magnitude of capacitive coupling between adjacent two of the resonators both vary when the relative positional relationship between the resonators varies, there arises a problem that it is difficult to adjust the characteristics of the band-pass filter.